There are four types of black holes and it all depends on their masses to differentiate between them. The first type is called a Primordial black hole. A Primordial black hole is the smallest compared to the other three types of black holes. As Primordial black holes are only hypothetical, there are only theories as to how they are formed. It is widely believed that these Primordial black holes were formed very early in the Universe right after the Big Bang. An important factor in its formation is the fluctuation in the density of the Universe.The second type is called a Stellar black hole. Stellar black holes have masses that can reach up to 20 times more than the mass of the sun. Initially, the core of a high mass star will completely burn to iron. At this point, energy production stops and the core of the star rapidly collapses resulting in a supernova explosion. If the core of the star is greater than about 2-3 solar masses (the maximum mass of a neutron star), the pressure of the neutrons will be unable to stop the collapse and thus, resulting in a stellar black hole. The third type is called a Supermassive black hole. These black holes can contain between a million and a billion times more mass than a typical Stellar black hole. There are only a handful of confirmed Supermassive black holes as most of them are too far away to be observed. However, according to astronomers, there is a Supermassive black hole located in the centre of the Milky Way. The name given to this black hole is “Sagittarius A*”. To put the size of Sagittarius A* into perspective, it has a mass equivalent to about 4 million suns. To this day, there are only theories of how Supermassive black holes are formed. Astronomers believe once a black hole is formed in the galaxy, it can grow by accretion of matter or merge with other black holes which in turn, forms a Supermassive black hole. However, a prevalent theory is that Supermassive black holes are formed out of the collapse of massive gas clouds during the early stages of the formation of the galaxy. The last type is called an Intermediate-mass black hole. Throughout history, astronomers believed that black holes only came in two sizes. The two being Stellar black holes and Supermassive black holes as mentioned earlier.  However, recent evidence from the Chandra X-ray Observatory, XMM-Newton(X-ray space observatory) and Hubble(Space Telescope) strengthens the case that mid-size black holes do exist. The first possible theory for the formation of Intermediate-mass black holes(IMBH) is that they form through the collapse of population III stars (the first generation of stars to form in the early stages of the universe). The second possible theory is that IMBHs are formed from collisions of massive stars or black holes in the interiors of star clusters.